It is important in springing sofas and loveseats, for example, that a person sitting at either end does not feel the unpleasant sensation of "lean-out" over the arm. "Lean-out" is caused by the difference between the strong spring support he gets underneath him on his inboard side and the virtual absence of support he gets from the absence of springs on his outboard side.
For over forty years, the presence of a high degree of "lean-out" has been particularly troublesome when springing sofas, loveseats, sectionals and similar furniture with sinuous springs of any type. The problem is even more bothersome in open-end modular styles, where there is no arm at either one end or both ends. Many attempts have been made to combat this problem but none have been fully successful, even when going to the extreme in labor and materials costs of placing two or even three sinuous spring bands at each arm end so close together as to virtually touch each other and as far out under the arm as possible. This construction is expensive because of the additional spring and labor required, it puts great additional strain on the furniture frame at the point of attachment of the additional spring, and it has the effect of slowing down the subsequent upholstering process.